Dump trucks have long been known in the art, for use in carrying different types of loads. One such use is in the construction industry for carrying dirt or other construction materials in the dump truck bed. A typical dump truck bed includes a tailgate defining a rear wall of the truck bed. The tailgate retains the material within the bed when the tailgate is in a storage or raised position and permits material to be ejected from the bed as the dump truck bed is raised. U.S. Pat. No. 5,456,521, assigned to the assignee of the present invention, discloses an unloading gate for a dump truck wherein the gate is mounted to one or more rails disposed on the bottom surface of the bed. The unloading gate moves from one end of the bed to the other to scrape and clean the bed after unloading or dumping of the material. The unloading gate in this patent is utilized in conjunction with a conventional dump truck bed which lifts at its forward end closest to the tractor cab and utilizes gravity to dump the material from the dump truck bed.
Applicant has had under development a new material carrying vehicle with a new material ejection system as is described in U.S. patent application Ser. No. 09/160,698, filed Sep. 25, 1998, entitled `Hydraulic System For a Work Machine Having an Ejector Cylinder and a Tailgate Cylinder`, assigned to the assignee of the present invention, which is hereby incorporated in its entirety herein. A variation of the above system is disclosed in U.S. patent application Ser. No. 08/936,128, filed Sep. 24, 1997, entitled `Tailgate Latching Mechanism` assigned to the assignee of the present invention, and hereby incorporated in its entirety herein.
Such a material eject system has a motor, for example, a hydraulic cylinder, that is connected to an ejector blade having a home position at the front end of the bed. To initiate a material eject operation, the motor moves the ejector blade toward the rear of the vehicle, thereby pushing the material in the bed off of the rear edge of the bed. The ejector blade is then retracted or returned to its home position at the front of the bed. The bed normally includes a tailgate which prevents material from exiting the bed while the material is being transported. During the material eject cycle, the tailgate is moved between its closed and open positions in synchronization with the operation of the ejector blade.
A vehicle having a material eject system has several advantages over traditional dump trucks. For example, the vehicle bed is not raised; and its center of gravity is not elevated as the material is being ejected. Therefore, vehicle stability is not adversely affected by the material ejection process. By maintaining a lower center of gravity, a material ejection system is more suitable for controlled spreading of the material during the ejection process. Further, by not raising the bed, the vehicle is not exposed to overhead obstacles such as trees and power lines. In addition, the profile of the ejector blade closely matches the cross-sectional profile of the bed and therefore, substantially all of the material in the bed is scraped out of the bed during the ejection process. Thus, a vehicle material ejection system is generally more efficient and flexible than the traditional elevating material dumping systems.
As previously mentioned, a material ejection system has the advantage of being able to eject material from the bed while the vehicle is moving without jeopardizing the stability of the vehicle. Further, the ejection of material is powered by a motor driven from the truck engine versus relying on gravity. Thus, the ejection is reliably continuous, uniform and generally repeatable. Given a constant vehicle speed, the material eject cycle results in a relatively uniform layer of material being spread over the surface. However, if the material is to be spread in layers of different thicknesses, either the speed of the vehicle must be varied or the speed of the ejector blade during the material ejection cycle must be varied. The vehicle is often operating in very rugged and uneven terrain; and therefore, the range of vehicle speed is very limited. Consequently, there is a need to provide a material eject cycle permitting the speed of the ejector blade to be varied. By permitting an operator to operate the ejector blade at different speeds, the operator can concentrate on moving the vehicle at a fixed speed and be able to spread the material being ejected therefrom at a uniform desired thickness.